Hello! My 12 week old has started to roll back to tummy first but not yet able to roll back the other way (from tummy to back). While we put him down to bed on his back, he always rolls onto tummy shortly after. This continues despite us trying to reposition him back onto his back whenever possible throughout the night. He also fusses and wakes whenever we do try to reposition him to his back. He seems to be breathing ok when he's in this downward tummy position, should we continue trying to repositon him back on his back? How do we help him learn to roll from tummy to back in need? We also have a secondary concern as when he's in this tummy position, he sleeps on his right arm so we are worried he is cutting off circulation to his arm. Hoping you can help as neither us or baby are sleeping well all the while trying to reposition him back to safer back sleeping! I will note that he's good with tummy time in the day once on his belly, neck strength is there but is starting to spread his arms wider and use his chest to lift rather than his arm strength. Thank you in advance.

Most babies will be rolling by 4-5months - some earlier, some later
This is very normal as babies grow & become stronger.
Many babies then prefer to sleep on their side or tummy.
This is not something that needs to be stopped. It is important to make his new environment as safe as possible.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

Is he in his cot now or still in bassinet & being wrapped/swaddled? He needs to be in his cot as he needs the space of the cot & arms free to roll safely,

As you will see from the article linked above, ensure the mattress is firm & flat with nothing loose or soft in the cot for now.
It is not unusual for baby to roll back to tummy first - soon he will be rolling both ways.
*Still always place him on his back for sleep, even tho’ he is rolling to tummy.

When you roll him back, does he wake (or just fussy?)
It would be helpful for him to have more tummy time - maybe get on the floor with him with some toys. Does he reach for toys while on his tummy?
Does he sometimes roll to his back during tummy time?
More tummy time (if possible) would be good to help increase upper body strength to toll well. (& both ways)
Once he is rolling both ways, still place him on his back for sleep & let him find his own position of comfort.
In the interim, try to gently roll him back if you are awake & he is on his tummy, but if he continues to protest, it is not achieving much.

No parent is expected to sit up all night & watch baby, so it is important to have his sleep environment as safe as is possible (as discussed above)

Increase tummy time should help with the rolling both ways.
Maybe chat with your Child Maternal Health Nurse or GP if you are concerned about him lying on his arm.